George T. Byrd

Novel characteristics of cassava, Manihot esculenta Crantz, a reputed C3-C4 intermediate photosynthesis species

The cassava plant, Manihot esculenta, grows exceptionally well in low fertility and drought prone environments, but the mechanisms that allow this growth are unknown. Earlier, and sometimes contradictory, work speculated about the presence of a C4-type photosynthesis in cassava leaves. In the present work we found no evidence for a C4 metabolism in mature attached cassava leaves as indicated i) by the low, 2 to 8%, incorporation of 14CO2 into C4 organic acids in short time periods, 10 s, and the lack of 14C transfer from C4 acids to other compounds in 12CO2, ii) by the lack of C4 enzyme activity changes during leaf development and the inability to detect C4 acid decarboxylases, and iii) by leaf CO2 compensation values between 49 and 65 μl of CO2 1−1 and by other infrared gas exchange photosynthetic measurements. It is concluded that the leaf biochemistry of cassava follows the C3 pathway of photosynthesis with no indication of a C3-C4 mechanism.However, cassava leaves exhibit several novel characteristics. Attached leaves have the ability to effectively partition carbon into sucrose with nearly 45% of the label in sucrose in about one min of 14CO2 photosynthesis, contrasting with 34% in soybean (C3) and 25% in pigweed (C4). Cassava leaves displayed a strong preference for the synthesis of sucrose versus starch. Field grown cassava leaves exhibited high rates of photosynthesis and curvilinear responses to increasing sunlight irradiances with a tendency to saturate only at high irradiances, above 1500 μmol m−2 s−1. Morphologically, the cassava leaf has papillose epidermal cells on its lower mesophyll surface that form ‘fence-like’ arrangements encircling guard cells. It is proposed that the active synthesis of sugars has osmotic functions in the cassava plant and that the papillose epidermal cells function to maintain a healthy leaf water status in various environments.

Relationships between specific leaf weight and mineral concentration among genotypes

Abstract Physiological functions are usually expressed on a leaf area basis, whereas leaf mineral concentrations are often expressed on a dry matter basis. If specific leaf weight (SLW; g DM m−2 leaf) differs among genotypes then variability in mineral concentration may depend on the basis of expression. Data from experiments with peanut (Arachis hypogaea L.) and pearl millet [Pennisetum glaucum (L.) R. Br.] lines and from the literature were used to examine relationships between leaf mineral concentration and SLW. Peanut and pearl millet were grown in pots in the greenhouse in soil and solution cultures. Specific leaf weight and ash and mineral concentrations were determined at the end of the experiments. Leaf ash concentration on a dry matter basis was negatively correlated with SLW and the correlation coefficients were significant in six of nine comparisons for the two species; r = −0.65 to −0.93. In the one peanut experiment in which mineral elements were determined, the correlations with ash were due mainly to correlations with Ca and Mg, while in pearl millet, correlations were due mainly to K. The slope of a plot of leaf constituents per unit of leaf area against SLW for a range of lines is a measure of the contribution of that leaf constituent to increased SLW. From data in the literature it appears that increased SLW is due mostly to the increase of cell wall components and nonstructural carbohydrates, and sometimes protein. Leaf mineral per unit of leaf area appears to be unrelated or only slightly increased with increased SLW and thus declines on a unit weight basis because of dilution by increased cell wall content or soluble carbohydrate.

Comparative effects of growth irradiance on photosynthesis and leaf anatomy of Flaveria brownii (C4-like), Flaveria linearis (C3–C4) and their F1 hybrid

Photosynthetic rates and related anatomical characteristics of leaves developed at three levels of irradiance (1200, 300 and 80 umol · m−2 · s−1) were determined in the C4-like species Flaveria brownii A.M. Powell, the C3–C4-intermediate species F. linearis Lag., and the F1 hybrid between them (F. brownii × F. linearis). In the C3–C4 and F1 plants, increases in photosynthetic capacity per unit leaf area were strongly correlated with changes in mesophyll area per unit leaf area. The C4-like plant F. brownii, however, showed a much lower correlation between photosynthetic capacity and mesophyll area per unit leaf area. Plants of F. brownii developed at high irradiance showed photosynthetic rates per unit of mesophyll cell area 50% higher than those plants developed at medium irradiance. These results along with an increase in water-use efficiency are consistent with an increase of C4 photosynthesis in high-irradiance-grown F. brownii plants, whereas in the other two genotypes such plasticity seems to be absent. Photosynthetic discrimination against 13C in the three genotypes was less at high than at low irradiance, with the greatest change occurring in F. brownii. Discrimination against 13C expressed as δ13C was linearly correlated (r2 = 0.81; P<0.001) with the ratio of bundle-sheath volume to mesophyll cell area when all samples from the three genotypes were combined. This tissue ratio increased for F. brownii and the F1 hybrid as growth irradiance increased, indicating a greater tendency towards Kranz anatomy. The results indicated that F. brownii had plasticity in its C4-related anatomical and physiological characteristics as a function of growth irradiance, whereas plasticity was less evident in the F1 hybrid and absent in F. linearis.

Photosynthetic Characteristics of Segregates from Hybrids between Flaveria brownii (C4 Like) and Flaveria linearis (C3-C4)

Characteristics related to C4 photosynthesis were studied in reciprocal F1 hybrids and F2 plants from Flaveria brownii (C4 like) and Flaveria linearis (C3-C4). The reciprocal F1 plants differed in 13C/12C ratios of leaves and the percentage of 14C initially incorporated into C4 acids, being more like the pollen parents in these traits. They did not differ in apparent photosynthesis or in O2 inhibition of apparent photosynthesis and differed only slightly in CO2 compensation concentration at 175 [mu]mol quanta m-2 s-1 and 400 mL L-1 O2. The 13C/12C ratios of 78 F2 progeny from the two F1 plants exhibited a normal distribution centered between those of the parents, with a few values slightly higher and lower than the parents. Apparent photosynthesis at 130 [mu]L L-1 CO2 and inhibition of photosynthesis by O2 was nearly normally distributed in the F2 population, but no values for F2 plants approached those for F. brownii (15.4 [mu]mol m-2 s-1 and 7.8%, respectively). Distribution of the CO2 compensation concentration measured at 1000 [mu]mol quanta m-2 s-1 and 400 mL L-1 of O2 in the F2 population was skewed toward F. brownii with 72% of the progeny having values <9 [mu]L of CO2 L-1 compared to 1.5 and 27.2 [mu]L L-1 for F. brownii and F. linearis, respectively. Correlations among traits of F2 plants were low (coefficients of 0.30 to -0.49), indicating that the C4- related traits are not closely linked in segregating populations. Plants in the F2 population selected for high or low apparent photosynthesis at 130 [mu]L of CO2 L-1 (six each) did not rank consistently high or low for 13C/12C ratios, O2 inhibition of apparent photosynthesis, CO2 compensation concentration, or activities of phosphoenolpyruvate carboxylase or NADP-malic enzyme. This study confirms results of earlier work that indicates independent segregation of C4 traits and also shows that the C4-like parental type can be recovered, at least for some characteristics (13C/12C ratio), in segregating populations. Recovery of fully functional C4 plants awaits further experimentation with C4 x C3 or C4 x C3-C4 hybrid plants that produce fertile progeny.

Leaf cavity CO2 concentrations and CO2 exchange in onion, Allium cepa L.

Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO2 that accumulates within their leaf cavities. Leaf cavity CO2 concentrations ranged from 2250 μL L−1 near the leaf base to below atmospheric (<350 μL L−1) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO2 concentrations with minimum values near midday and maximum values at night. Conductance to CO2 from the leaf cavity ranged from 24 to 202 μmol m−2 s−1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO2 was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO2 concentrations and conductance values or as measured directly by 14CO2 labeling experiments. The photosynthetic responses to CO2 and O2 were measured to determine whether onion leaves exhibited a typical C3-type response. A linear increase in CO2 uptake was observed in intact leaves up to 315 μL L−1 of external CO2 and, at this external CO2 concentration, uptake was inhibited 35.4±0.9% by 210 mL L−1 O2 compared to 20 mL L−1 O2. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO2 and greatly restrict the refixation of leaf cavity CO2 by photosynthetic tissue.